Hopes of a deal to send thousands of migrants back from Europe to Turkey are hanging in the balance as tense negotiations continue in Brussels.
It is understood that a position agreed in the early hours by the EU's 28 national leaders waters down the terms of a plan outlined last week, which held out the prospect of visa liberalisation for Turkish nationals and the reopening of long-stalled talks on accession to the EU.
Turkish prime minister Ahmet Davutoglu cautioned he was not in Brussels for "bargaining" as he arrived for a breakfast meeting with European Council president Donald Tusk to discuss the EU proposals, stressing that "humanitarian values" should determine the response to the crisis, which has seen his country take in almost three million refugees from the civil war in neighbouring Syria.
Meanwhile, in Ankara, Turkey's president Recep Tayyip Erdogan was scathing about the stance of EU nations in a televised address, saying: "At a time when Turkey is hosting three million, those who are unable to find space for a handful of refugees, who in the middle of Europe keep these innocents in shameful conditions, must first look at themselves."
The package drawn up last week envisaged Europe taking one Syrian refugee from the camps in Turkey for each irregular migrant returned to the country after arriving in Greece, up to a maximum of 72,000. Turkey was also demanding visa-free travel within the EU's Schengen borderless area - which does not include the UK - and talks on a number of "chapters" in its application for EU membership. EU leaders were also asked to double to six billion euro (£4.7 billion) the financial support agreed late last year.
But a number of EU states - particularly Cyprus - are wary of any move towards membership for Turkey, and it is understood that Thursday's talks added a list of conditions for Ankara before any liberalisation of visas.
French president Francois Hollande said he could not guarantee "a happy outcome" to the two-day summit, while Belgium's Charles Michel said he would prefer "no deal to a bad deal with Turkey".
Mr Michel said it was important to "defend European values" and - in an apparent reference to the Turkish state's seizure of control over a critical newspaper - added: "Freedom of speech and expression are essential to democracies. Belgium is committed to defend these values at home and abroad."
As the summit agenda was extended to allow protracted talks with Mr Davutoglu on the latest EU offer, David Cameron chaired a meeting of leaders from Germany, Italy, France, Spain and Malta, as well as High Representative on Foreign Affairs Federica Mogherini, to discuss the threat of a renewed surge of migrants attempting the Mediterranean crossing from Libya to Italy over the summer.
The Prime Minister wants the EU to open talks with the newly established government of national accord in Libya to expand its anti-trafficking operation into the North African state's territorial waters. At present, EU ships - including one Royal Navy vessel - are able to intercept migrant boats in international seas, but Mr Cameron believes that taking action closer to the Libyan coastline will improve its chances of turning boats back and deterring migrants from setting off at all.
Some 1.2 million migrants arrived in Europe over the course of last year, and while the bulk of them took the shorter sea route from Turkey to the Greek islands, more than 150,000 attempted the more dangerous crossing from Libya. Mr Cameron said the EU must not "take its eye off the ball" in the central Mediterranean because of the focus on Turkey.
Mr Cameron's official spokeswoman said there were still "difficult issues to work through" on the Turkish deal, but that there was a general sense that leaders wanted to reach agreement. "It's in all our interests that we work together to try to make this plan work, to break the smugglers' business model and deter migrants from coming," she said.
Mr Davutoglu said he was hopeful of an agreement that would help Syrian refugees as well as deepening his country's relations with the EU.
"Turkey has received 2.7 million refugees without any significant assistance from anywhere," said the Turkish PM.
"Today I want to re-emphasise that Turkey will continue her policy to have this attitude of a humanitarian perspective.
"The EU and Turkey have the same goal, the same objective, to help Syrian refugees especially and also to have a new future in our continent in a right manner.
"I am sure - I hope - we will be achieving our goal to help all the refugees as well as to deepen Turkey-EU relations, which is good news for our continent and for humanity altogether."
The proposed deal with Turkey was denounced by the Refugee Council as "immoral, unworkable and probably illegal".
Amnesty International refugee director Steve Symonds said: "The plan lacks both logic and compassion. By abandoning their legal obligations, European leaders won't put a stop to refugee migration."
But Mr Cameron's spokeswoman said the UK believes the proposed new arrangements can be imposed "in line with EU and international law" as individual cases would be assessed on their merits.
The PM has signalled Britain's readiness to supply further border officials to help Greece process new arrivals, who look likely to be offered individual interviews and appeals to make their case for asylum in the EU or face return to Turkey.
The intention is to have the system in place soon after agreement is reached, to limit any surge in migrants hoping to avoid the new arrangements.